1. Technical Field
The present invention pertains to a liquid crystal display device and particularly relates to a liquid crystal display device that is equipped with a liquid crystal display panel where alignment control ability is imparted to an alignment film by irradiation with light.
2. Related Art
Liquid crystal display devices have a TFT substrate on which pixel electrodes and thin film transistors (TFTs) are formed in a matrix, an opposing substrate that opposes the TFT substrate and on which color filters are formed in places corresponding to the pixel electrodes of the TFT substrate, and liquid crystal that is held between the TFT substrate and the opposing substrate. Additionally, liquid crystal display devices form an image by controlling, per pixel, the transmittance of light by the liquid crystal molecules.
Liquid crystal display devices are flat and lightweight, so they are used for various purposes in many different fields, from large display devices in televisions and the like to mobile telephones and digital still cameras (DSC). On the other hand, viewing angle characteristics of liquid crystal display devices are a problem. Viewing angle characteristics are a phenomenon where brightness varies or chromaticity varies between when the screen is seen from the front and when the screen is seen from a diagonal direction. In-Plane Switching (IPS), where the liquid crystal molecules are moved by a horizontal direction electric field, has excellent viewing angle characteristics.
As a method of aligning an alignment film that is used in liquid crystal display devices, that is, imparting alignment control ability, the related art includes a method of aligning the alignment film by rubbing. This is a method where alignment is performed by rubbing the alignment film with a cloth. On the other hand, there is a technique called photoalignment where alignment control ability is imparted to the alignment film without contact. IPS does not require a pretilt angle, so photoalignment can be applied thereto.
In JP-A-2004-206091, there is disclosed photodegradative photoalignment by irradiation with light represented by ultraviolet light. According to this, photodegradative photoalignment (1) reduces alignment disorder caused by the complex step structure of the pixel portions and (2) resolves display defects caused by alignment disorder resulting from dust or disorder of the bristle tips of the rubbing cloth and breakage of the thin film transistors resulting from static electricity during alignment by rubbing and resolves the cumbersomeness of the process of frequent replacement of the rubbing cloth that is needed in order to obtain homogenous alignment control ability.
However, it is known that, in regard to the point of imparting alignment control ability to the alignment film, alignment stability is usually lower in photoalignment in comparison to alignment stability in rubbing. When alignment stability is low, the initial alignment direction fluctuates, which leads to display defects. Particularly in liquid crystal display devices that use an in-plane switching liquid crystal display panel where high alignment stability is demanded, it is easy for display defects symbolized by afterimages to occur as a result of alignment stability being low.
In photoalignment, there is, in the LCD process, no step of stretching the main chain of a polymer into a straight line as there is in rubbing. For that reason, in photoalignment, the main chain of a synthetic polymer alignment film represented by a polyimide that has been irradiated with polarized light is broken in a direction parallel to the polarization direction, whereby uniaxiality is imparted. The liquid crystal molecules align along the direction of the long main chain that remains extending on a straight line without being broken, but when the length of this main chain becomes short, uniaxiality drops, interaction with the liquid crystal becomes weak and alignment deteriorates, so it becomes easy for afterimages to occur.
Consequently, in order to improve the uniaxiality of the alignment film and improve alignment stability, it is necessary to increase the molecular weight of the alignment film. As a technique for solving this, a photoalignment film material obtained by imidizing polyamide acid alkyl ester can be used. According to this, in the polyamide acid alkyl ester material, the molecular weight can be kept large even after imidization without being accompanied by a decomposition reaction to an anhydride and a diamine at the time of the imidization reaction that had occurred in the conventional polyamide acid material, and alignment stability on a par with rubbing can be obtained.
Further, the polyamide acid alkyl ester material does not include carboxylic acid in its chemical structure, so its LCD voltage holding ratio rises in comparison to the polyamide acid material, and an improvement in long-term reliability can also be ensured.
In order to obtain alignment stability and long-term reliability of a photoalignment film, application of the polyamide acid alkyl ester material is effective, but this material usually has high alignment film specific resistance in comparison to the polyamide acid material. For that reason, when a direct-current voltage superposes on the signal waveform that drives the liquid crystal molecules and becomes residual DC, the time constant until the residual DC eases is large and it becomes easy for this to lead to image persistence (DC afterimages).